This invention relates to displays for the study of a characteristic associated with a well, and more particularly to a method and apparatus for producing a graphic recording of a characteristic of the periphery of a well, such as for example the quality of the cement bond around a casing.
Oil well completion operations for initial production begin with the placement of a casing and the injection of cement between the casing and the surrounding earth formations. The main function of the cement is to isolate the formations so as to avoid the flow of fluids along the casing between aquifer layers and other formations containing hydrocarbons. It is thus very important to know the bonding quality of the cement around the casing.
A logging apparatus for obtaining this information is described in U.S. Pat. No. 4,255,798, issued Mar. 10, 1981 to Havira, which hereby is incorporated herein by reference. In one embodiment, the downhole apparatus comprises several (for example, eight) acoustic transducers positioned helically around the body member for the inspection of the entire casing periphery. A signal is generated that is representative of the bonding quality of the cement on the part of the casing corresponding to the exploration field of this transducer.
One advantage of this apparatus is that it allows the study of the bonding of the cement on surfaces of limited size on the periphery of the casing, thereby making it possible to identify communication channels between certain formations even when the average quality of the cement appears to be good. One of the difficulties encountered has to do with the recording of the results. In fact, if a conventional plot on a film medium is made of the output signal as a function of depth, a number of curves are obtained which are not directly utilizable and which are often even difficult to separate from each other. In addition, it is desirable to provide other data, for example the average quality of the cement, thus further complicating the representation.
Logging techniques for furnishing a continuous visual representation of the wall of the borehole or of the formation layers around it are described in Canadian Pat. No. 685,727 (Mann & Threadgold). Signals coming from current electrodes are used for the intensity modulation of an oscilloscope. The purpose is to obtain an image of the limits between the formation layers in order to deduce the dip; accordingly, the boundaries between the changes in the recording shade give the desired information of the layer limits. The shades themselves, which correspond to resistivity values, are strictly speaking of no interest. In addition, a spot whose intensity is modulated in proportion to the amplitude of the signal does not give a satisfactory distribution of shades for the objects of this invention.
One advantageous technique for the graphic presentation of measurements of the bonding quality of the cement between a casing and the wall of a borehole is described in copending application Ser. No. 304,887, filed Sept. 23, 1981. A plurality of measurements are carried out as a function of depth in directions distributed uniformly around the axis of the well, as taught in the aforementioned Havira patent. These measurement signals are converted into respective quantified signals having a limited number of possible values. A shading code is selected for each such measurement signal based on the quantified value thereof. For each depth level, a line modulated by the selected shading codes is displayed, producing over a plurality of depth levels parallel longitudinal bands. Hence, each band comprises a succession of shaded zones, the darker zones representing regions of high compressive strength, i.e. good bonding quality.
While this form of presentation provides a relatively clear picture of the condition of the periphery of the well, the presentation tends to take on a somewhat artificial or excessively geometrical appearance due to uniformity of the pattern for each value of the characteristic. Moreover, the graphic transitions between different values of the characteristic measured for neighboring zones of the well can appear relatively harsh.